The Flash file would have a movieclip in it. Give that movie clip an instance name of MyRenderTarget_mc. That movieclip should contain a PNG file - any image will do as it will be replaced. The PNG will end up being replaced by the render texture in the game. The PNG file should have its linkage setup to 'Export for ActionScript' and 'Export in frame 1' and the Identifier for this texture should be set to MyRenderTarget. (These things are found by right clicking the PNG in the Library panel and choosing properties.)

In your level, you would set up a SceneCapture2DActor and point it at your object. Create a new TextureRenderTarget2D in the Content Browser and name it to what you've specified in Unrealscript. Select the new render texture, then go into the Texture Target field of the capture actor, and insert TextureRenderTarget2D'MyPackageName.MyRenderTexture' by pressing the green arrow.

My External Texture is rendering transparently!

Select the movie clip containing the texture to be replaced by the SceneCapture2DActor image.